Organic solvent and liquid agent of high vapor pressure in particular tend to condense within an evaporator on account of a slight variation of the atmospheric temperature. Accordingly, it is required that condensation of the liquid agent be effectively removed so that clean vapor can be supplied stably and reproducibly in this type of evaporator. In addition, corrosive and flammable chemical vapor should be supplied stably and reproducibly in the evaporator.
In the meanwhile, there are in general two systems for controlling flow rate of vapor supplied from the evaporator, i.e., a carrier control system and a liquid material direct control system. The former system uses an inert gas of nitrogen or the like as a carrier gas which indirectly causes the liquid material to evaporate thereby controlling the flow rate of the vapor, while the latter system directly controls the flow rate of the liquid material or the flow rate of the vapor itself generated from the liquid material.
One example of the evaporator (vaporizer) of the carrier control system is a carburetor used in an internal combustion engine. In the carburetor, the carrier gas introduced from a wider mouth is compressed at a narrower part (venturi), and then is reduced in pressure during expansion. The pressure-reduced gas causes the liquid agent to be introduced into a nozzle from a tank, whereby the liquid agent is sprayed from the nozzle so as to be evaporated quickly under the reduced pressure.
There is another method in which the vapor of the liquid agent can be obtained by enhancing evaporation of liquid agent by means of heating of the liquid agent. Such method can be implemented in two ways. One way is to supply vapor not by force but by pressure of the generated vapor, and the other way is to supply vapor by introducing a certain amount of carrier gas into the liquid agent tank.
Another example of the carrier control system is shown in FIGS. 1 and 2, respectively. An evaporation amount of the liquid agent is determined by a temperature and a saturated vapor pressure of the liquid agent. In the evaporator shown in FIG. 1, a liquid agent 2 is evaporated at a room temperature within a liquid agent tank 1 formed by a sealed container, and then a certain amount of carrier gas controlled by a massflow controller 3 is introduced into liquid agent tank 1. More particularly, the vapor of the liquid agent whose evaporation amount is determined by the temperature of liquid agent and the vapor pressure thereof is discharged from liquid agent tank 1 through a vapor outlet (not shown) by means of the carrier gas in the evaporator shown in FIG. 1 to be supplied to an etching chamber or the like.
In an evaporator shown in FIG. 2, a liquid agent 5 is accommodated within a liquid agent tank 4 formed by a sealed container, and then a certain amount of carrier gas whose amount is controlled by a massflow controller 6 is blown into a liquid agent 5, whereby liquid agent 5 is bubbled to be evaporated. Thereafter, the vapor of liquid agent is discharged from liquid agent tank 4 together with the carrier gas through a vapor outlet (not shown) to be supplied to an etching chamber or the like.
In the first example of the evaporator of the material direct control system, a certain amount of liquid agent whose with amount is controlled by a liquid massflow controller is fed into a heating evaporator thereby heating the liquid agent to evaporate. The second example of such an evaporator controls the flow rate of vapor directly by providing a massflow controller in a piping disposed on the vapor outlet side of the evaporator shown in FIG. 2.
Appropriate ones of the above-described systems of the evaporators are selected depending on such factors as the property of liquid agent, the precision required for controlling flow rate, and the cost. When the vapor generated by evaporating the flammable and corrosive liquid agent is used, the range of useable control systems is limited depending on the material of evaporators and the necessity of heating operation. In addition, the material direct control system cannot be adopted in this case, since the use of the vapor massflow controller is prohibited in controlling the flow rate of corrosive vapor.
Therefore, the system adopted to control the flow rate of vapor of the flammable and corrosive liquid agent is restricted to one using evaporators shown in FIGS. 1 and 2. In particular, the evaporator shown in FIG. 2 is frequently used because of its simple structure and, property of generating the vapor of a higher concentration than that allowed in FIG. 1, alike. However, the evaporator shown in FIG. 2 is likely to generate mist on account of bubbling. For instance, if the vapor of liquid agent is used in etching, mist mixed in the vapor might attach to an etched surface. Accordingly, an etching rate becomes greater in a part provided with mist than in other parts, whereby etching cannot be carried out uniformly. On the other hand, from the viewpoint of material, it is difficult to use a mist trap on account of handling of corrosive liquid agents. Thus, the evaporator shown in FIG. 2 cannot be used for etching without a means for completely removing mist from vapor. In this respect, the evaporator shown in FIG. 1 has been used in such a processing as etching.
There is, nevertheless, a disadvantage in the evaporator shown in FIG. 1 in that liquid agent condenses within liquid tank 1. In the case of using the liquid material of a high vapor pressure, in particular, the material is likely to condense on account of a slight variation of the room temperature or the temperature of liquid agent. Droplets caused by such condensation are liable to attach to an inner surface of an upper cover of liquid tank 1, and particularly large droplets are liable to attach to the cover at the periphery thereof. If droplets attach to the inner surface of the upper cover of liquid tank 1, an area to contact the liquid agent with the carrier gas is changed thereby altering an evaporation area of the liquid agent. Accordingly, the evaporation amount of liquid agent is varied, which disables precise control of the vapor of liquid agent, so as not to effect supply of the vapor with a constant flow rate desired. Thus, the droplets attaching onto the inner surface of the upper cover of liquid agent tank 1 need to be removed in order to control precisely the flow rate of vapor of the liquid agent supplied from the evaporator.
The droplets attaching to the inner surface of the upper cover of liquid agent tank 1 shown in FIG. 1 can be removed either by removing the droplets physically by means of, e.g., a wiper, or by removing the droplets by evaporation using a carrier gas. In the case of using the wiper or the like, it is very much likely that particles are generated. In addition, the droplets cannot be removed effectively by only blowing the carrier gas into liquid agent tank 1 as can be seen from FIG. 1. As a result, the time required for removal of droplets becomes longer, while evaporation of liquid agent 2 accommodated within liquid agent tank 1 proceeds. This means that the liquid agent is consumed more than necessary in accordance with a droplet removal operation. Further, in order to compensate for decrease in temperature of liquid agent 2 on account of evaporation, a time-consuming operation of temperature adjustment is required for stabilizing the temperature of liquid agent 2 again, before supply of vapor of the liquid agent is started at a constant flow rate from the evaporator. Thus, there are many problems in the method of simply blowing the carrier gas into liquid agent tank 1.